Materials for packaging foods and various articles often are required to have a gas barrier property, particularly, an oxygen barrier property. Such properties protect packaged contents from influence such as oxidation deterioration due to oxygen. Packaging foods, in particular, has a problem of proliferating microorganisms due to the presence of oxygen and thus decaying the contents. For this reason, conventional packaging materials are provided with gas barrier layers to prevent gas, such as oxygen, from permeating through them.
Examples of such gas barrier layer include metal foil or a vapor deposition layer of metal or a metal compound. Generally used gas barrier layers include aluminum foil, an aluminum vapor deposition layer, a silicon oxide vapor deposition layer and an aluminum oxide vapor deposition layer. Metal layers such as aluminum foil and an aluminum vapor deposition layer, however, have disadvantages of not showing the packaged contents and having low disposability, for example. Metal compound layers such as a silicon oxide vapor deposition layer and an aluminum oxide vapor deposition layer have disadvantages of loosing the gas barrier property considerably by deformation or drop of the packaging material or impact to the packaging material while transported.
The gas barrier layer also may employ a layer formed of a vinylalcohol based polymer with excellent gas barrier property such as polyvinyl alcohol and an ethylene-vinyl alcohol copolymer. Such a layer formed of a vinylalcohol based polymer has advantages of being transparent and having less problems on disposability; it is applied to a wider range of uses.
The vinylalcohol based polymer exhibits the gas barrier property by crystallizing through hydrogen bonds between hydroxyl groups in the molecules. Accordingly, conventional vinylalcohol based polymers show a high gas barrier property when they are dried, whereas they tend to have loosened hydrogen bonds and show a lowered gas barrier property when they have absorbed moisture due to water vapor, for instance. Thus, vinylalcohol based polymers such as polyvinyl alcohol do not easily exhibit a high level of gas barrier property under a highly humid condition.
In addition, studies have been made on materials containing a polymer compound and a hydrolyzed and condensed product of metal alkoxide (for instance, tetramethoxysilane) as materials with a gas barrier property. Such materials are disclosed in, for example, JP2002-326303 A, JP7 (1995)-118543 A, and JP2000-233478 A.